1. Field of the Invention
This invention relates to a signal recording method, a signal recording device, a recording medium and a signal processing method for digital audio data.
2. Description of the Related Art
In a format for a compact disc, as a digital audio disc, the sampling frequency is set to 44.1 kHz, so that the maximum reproducible frequency is set at 22.05 kHz. The quantization bits are set to 16 bits, with the dynamic range being approximately 98 dB.
Meanwhile, a sound source, such as gamlan or yodel-voice, is known to contain a significant amount of frequency components exceeding the maximum reproducible frequency of the digital audio disc such that the sampling frequency of 44.1 kHz is insufficient.
Recently, the digital/analog (D/A) converter is improved in accuracy and now can handle signals of weak intensity, such that the dynamic range of approximately 98 dB obtained for the quantization bits of 16 bits of the digital audio disc also has become insufficient.
Thus it may be contemplated to increase the sampling frequency or the number of quantization bits beyond those used in the conventional practice to improve the sound quality to produce broad frequency band or broad dynamic range signals for recording on a disc (recording medium) of the size of the conventional digital audio disc.
It is technically possible to record signals sampled with the sampling frequency higher than 44.1 kHz on a disc of the size of the conventional digital audio disc or to record data having the number of quantization bits larger than 16 by narrowing the track pitch on the recording disc or by shortening the wavelength of the laser light beam of the optical pickup.
However, if the device or the recording medium is modified for raising the recording capacity for improving the sound quality, it becomes impossible to maintain interchangeability with respect to the format of the conventional digital audio disc thus confusing the software market.
The present Assignee has proposed in the Japanese Patent Application 7-147742 (1995) a method of recording a sound source exceeding the conventional frequency band without modifying the format of the conventional digital audio disc for maintaining interchangeability. The frequency band herein means not only the usual frequency band but also the band of the quantization noise in S/N. The signal recording method proposed in this prior art resides in encoding the playback range of the conventional digital audio disc by straight PCM and in encoding the remaining areas as sub-areas by high-efficiency encoding. The signal portion in the playback range of the conventional digital audio disc inaudible to the human ear is detected for synthesizing the signal information of the above-mentioned sub-signals thereon. In this case, if the signal recorded by the above method is reproduced in accordance with the format of the conventional digital audio disc, the signal components of the conventional frequency band can be reproduced. If a playback device used can reproduce the sub-area as well, the reproduced sound can be improved in sound quality.
This method cannot be applied satisfactorily in case the volume of the sub-area signals is large since then the information cannot be recorded in its entirety.